1. Field of the Invention
The invention relates to the field of high frequency communications, an in particular to a complementary metal-oxide-semiconductor high frequency amplifier.
2. Related Art
In a conventional high-frequency amplifier, a bipolar transistor (or transistors) is used to provide the desired signal gain, while also providing the responsiveness required to maintain signal integrity. However, as high-frequency amplifiers become more common in consumer goods (e.g., a radio-frequency (RF) amplifier in a cellular telephone), reducing the price of those amplifiers becomes increasingly important. One way to reduce costs is to implement the amplifier using a metal-oxide-semiconductor (MOS) or complementary MOS (CMOS) process instead of the more expensive bipolar process.
FIG. 1 shows a conventional MOS RF amplifier 100. MOS amplifier 100 includes an input terminal 101, an output terminal 102, capacitors C1 and C2, resistors R_UP, R_DN, and R_SET, and an NMOS transistor 110. Capacitor C1 is coupled between input terminal 101 and the gate of transistor 110, while capacitor C_OUT is coupled between the drain of transistor 110 and output terminal 102. Resistors R_UP and R_DN are serially coupled between a supply voltage VDD and ground, with the gate of transistor 110 being connected to the junction between the two transistors. Finally, resistor R_SET and transistor 110 are serially coupled between supply voltage VDD and ground.
During operation, an input RF signal V_IN applied to input terminal 101 is filtered of any DC component by capacitor C1 and the AC signal is provided to the gate of transistor 110. Meanwhile, resistors R_UP and R_DN form a voltage divider that applies a bias voltage to the gate of transistor 110. By properly sizing resistors R_UP and R_DN, the bias voltage can be sized such that transistor 110 operates in its linear region in response to the AC signal from capacitor C1. Consequently, transistor 110 can apply gain without clipping or otherwise distorting the signal (so long as the input signal is not large enough to force transistor 110 into its saturated region).
In response to the AC signal at its gate, transistor 110 adjusts the magnitude of the current flow through resistor R_SET, which in turn generates an output signal at the source of transistor 110. Since the voltage drop across resistor R_SET is equal to the current flow times the resistance of resistor R_SET, the range of the output signal at the source of transistor 110 can be set by selecting an appropriate resistance for resistor R_SET. Increasing or decreasing the resistance of resistor R_SET increases or decreases, respectively, the output range of amplifier 100.
The amplified output signal at the source of transistor 110 is then filtered by capacitor C2 of any DC component that might have been introduced during the amplification process. The AC signal is then provided as an output signal V_OUT at output terminal 102.
In this manner, amplifier 100 provides a relatively simple means for RF amplification using a CMOS implementation. However, because current is always flowing through the voltage divider formed by resistors R_UP and R_DN, amplifier 100 can exhibit excessive power consumption. This power inefficiency is generally undesirable, and can be particularly problematic in devices that run off of a self-contained power supply (a battery). For example, using amplifier 100 in a cellular telephone to reduce the overall cost of the phone may result in an unacceptable decrease in talk time for that phone.
Accordingly, it is desirable to provide a power-efficient, high frequency CMOS amplifier.